Embodiments of the invention relate generally to power electronics for exhaust after-treatment component heaters and more particularly to power electronics for control and monitoring of electronic catalysts and decomposition elements associated with reductant delivery exhaust after-treatment.
There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to have improved emissions while having a maximum of fuel economy. Those pressures apply to engines fueled with gasoline, diesel, natural gas, or with any other alternative fuels such as hydrogen, ethanol, or additional bio-fuels.
Dividing the types of components into three distinct categories serves to simplify the explanation of the locations for an exhaust after-treatment heater. The three types of components are: the 3-way catalyst, the particulate filter, and the reductant decomposition tube. The 3-way catalyst combines the undesirable hydrocarbon and carbon monoxide emissions with excess oxygen in the exhaust stream and catalyzes an oxidation reaction where water and carbon dioxide are the output. Further, a reduction reaction occurs where nitrogen oxides, or NOx emissions, are reduced to nitrogen and oxygen. Historical 3-way catalyst systems enrich the combustion such that the combustion continues at a low level inside the exhaust system to more quickly raise the temperature of the catalyst, typically referred to as “catalyst light-off.”
The particulate filter, for diesels is the Diesel Particulate Filter (“DPF”). This component is a filter that traps carbon particulates or soot. The filter “loads up” with the particles that are being trapped because the filter's pores are smaller than the particles. Eventually, the back-pressure caused by a loaded filter flags the regeneration of this filter. The regeneration is accomplished by heating the filter material, typically ceramic, to such a high temperature that the carbon particulates burn off in the presence of excess oxygen. This heating is typically accomplished by enriching the exhaust with unburned fuel that then burns at the filter, thereby heating it.
The reductant decomposition tube is where urea-water solution is added by a Reductant Delivery Unit (“RDU”) to the exhaust stream. This urea-water solution aids in Selective Catalyst Reduction (“SCR”) by decomposition of the urea into ammonia and water. This ammonia then reduces nitrogen oxides into diatomic nitrogen and water. Typically, hot exhaust gas is expected to decompose the urea into ammonia and water inside the decomposition tube. This is, however, not always efficient as urea decomposes over a narrow temperature range into ammonia and water, and more frequently decomposes in additional reactions to deposits that do not contribute to SCR.
During engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions, due, at least in part, to cold exhaust after-treatment components. The worst emissions are during the first few minutes of engine operation, after which the catalyst, other exhaust components and engine approach operating temperature.
A number of pre-heating methods have been proposed, most of which involve additional combustion products to be made. The fastest method to heat a catalyst, decomposition element, or particulate filter is directly with electrical power. Electrical energy is converted to heat inside a component suitable in geometry and material to be heated by the Joule or Ohm losses that are caused by the flow of current through that component. As such, it is desirable to know the temperature of the heater and to control that temperature.
Because the heating technique uses an electrical current, the system includes electronics for providing an appropriate excitation to the component in the exhaust system. This excitation may include controlling the electrical energy and determining when that electrical energy is applied.
Conventional resistive heating is accomplished open-loop, or without control of electrical energy based on a temperature. A remote thermostat or computational model may be incorporated to provide some control to prevent a runaway temperature event and some level of control. More sophisticated methods may monitor the current through the heater to estimate the temperature or direct thermocouple, positive/negative temperature coefficient sensor, or other means for determining the temperature for a more precise regulation of component temperature.
The metallic component that is heated will have a positive temperature coefficient of resistance to electrical current (i.e., its electrical resistance will increase as its temperature increases). Ideally, knowing the initial resistance and final resistance would allow the temperature of the component to be known with some degree of precision. The best metals for resistive heaters usually have very small positive temperature coefficients and therefore measurement of the change in resistance by only monitoring current will be desensitized by harness resistance and aging of numerous interconnecting components. Additionally, electronic catalysts, or E-cats, are made of stainless steel and also suffer from a small temperature coefficient of the material. Therefore, it becomes difficult to distinguish a change in resistance of the heater component from a change in resistance of other components connected in series.
It would be advantageous to more precisely know the resistance change of the heater component such that control of the temperature may be accomplished.